1. Field of the Invention
The present invention relates to telephone networks and more particularly to private telephone networks which use a public telephone network for limited periods to route some calls.
2. Description of the Prior Art
Using a public network in this way is known to be economically justified in some private networks because the overall cost is less than the investment in the private network that would have been required to enable it to route all the required calls internally. To enable the public network to be used whenever necessary, the manufacturer of the private network incorporates into it resources which are sometimes referred to as a xe2x80x9cdynamic linkxe2x80x9d. A dynamic link is able to set up a xe2x80x9cmediumxe2x80x9d for a call on which voice or data signals of the call are exchanged between two nodes of the network. While it exists, the medium uses resources of the public network at least over an intermediate part of its path. The proprietor of the private network is billed for such use, the cost of which increases with the time for which the resources are used. This is why the link is a xe2x80x9cdynamicxe2x80x9d link and able to set up and clear down the medium.
The medium is typically set up each time there is a call to be set up on the medium between the two nodes and the medium has previously been cleared down. It is cleared down whenever there is no longer any call in progress.
A dynamic link can be a xe2x80x9cmultichannelxe2x80x9d link. The medium that it sets up is then used to route several telephone calls simultaneously between the two nodes connected by the link.
It has been found that the medium of a multichannel dynamic link is sometimes retained unnecessarily because private network users forget to terminate calls using the medium. This is why a prior art method of operating a link of this kind seeks to obtain a financial advantage by limiting the duration of such unnecessary retention. In the prior art method, if only one call is in progress on the medium, the medium is cleared down when the time which has elapsed since the start of the call reaches a predetermined authorized duration, such as one hour. If several calls which were started separately are in progress simultaneously, the prior art method clears down the medium if the time that has elapsed since the most recent start reaches the authorized duration.
The prior art method has an advantage if a call reaches the authorized duration at a particular time although, from the technical point of view, it would be beneficial for it to be able to exceed that duration and at the same time there is another call in progress which has a duration less than the authorized duration. The advantage is that, in this case, clearing down, which then has no financial advantage, is prevented for the first of the two calls.
However, the prior art method has the disadvantage that in the presence of a first call of the above kind the medium is too often retained for an excessively long time, and possibly indefinitely, because of the setting up of other calls which have durations very much less than the authorized duration and between which there are intervals during which only the first call continues.
A general object of the present invention is to limit the operating cost of a multichannel dynamic telephone link with limited risk of interrupting a useful call.
A particular object of the present invention is to retain the above advantage of the prior art method but to avoid the above disadvantage thereof.
To achieve the above objects, the invention provides a multichannel telephone link including means for provisionally setting up an internode medium connecting two nodes of a telephone network and including a plurality of channels each adapted to carry a call between the two nodes, a plurality of time-delayed controllers associated with respective channels and each adapted to assume a measurement state or an overshoot state or a latent state according to whether a call has been supported by the channel associated with the controller for a time less than an authorized duration or a call has been supported by the channel for a time at least equal to the authorized duration or no call has been supported by the channel, respectively, and means for clearing down the internode medium when none of the time-delayed controllers is in its measurement state.